"If you're paying attention to what's going on in the design world, you've probably noticed the ongoing debate around skeuomorphism vs. flat design." Good overview of the subject from Sacha Greif. This is a very important point: "But where the main victim of realism is merely good taste, taking minimalism too far can have serious consequences on usability. Users have come to rely on a lot of subtle clues to make their way through an interface: buttons have slight gradients and rounded corners, form fields have a soft inner shadow, and navigation bars 'float' over the rest of the content. Remove all these clues, and you end up with a flat world where every element is suddenly placed at the same level, potentially leading to confusion: Is this a button, or simply a banner? Will anything happen if I tap this?"

When people say "skeuomorphism is bad", what they're really saying is "woodgrain-patterned plastic cases on electronics are tacky".

In some cases, not in all. In the former, they are using terminology ineffectively and aren't helping their case. In the latter, they are making an extreme case that is not well supported.

Skeuomorphism an extreme on a continuum, as is flat design.

No, Skeuomorphism is not the extreme at the end of the continuum -- it is it's own whole spectrum. There are innumerable instances of skeuomorphism even in "flat" designs like Metro and Holo.

Also, really? Checkboxes are simply digital mimicks of things in the real world? I see them as almost identical digital copies. What if I used a Wacom Cintiqu to check a digital checkbox. It wouldn't be skeuomorphic, it'd be a checkbox on a digital page.

Why is it a checkmark at all? It could be yes/no text, or a red or green circle, or any number of non-analogous objects. Why a checkmark instead of a tick, or a dot, or an X (which it may be)? Yes, trying to make a reference to something that you are already familiar with in some other context is the definition of skeuomorphism. Whether the analog is very close to its digital counterpart or not very similar at all doesn't change that it is skeuomorphism.